[Simple] Proposed applicability statement for MSRP

[Robert Sparks <RjS at xten.com>]

Folks -

One of the things that Sam's review highlights is the need for a more 
explicit applicability statement in
the MSRP specs. Here's a proposal for the text for such a statement. 
(We'll add references in the
places that need them when we fold the result of this conversation into 
the spec).

RjS

---------------------------------------------------------
Applicability Statement:

MSRP is not designed for use as a standalone protocol. MSRP MUST only 
be used in the context of a rendezvous mechanism meeting the following 
requirements:

   - The rendezvous mechanism MUST provide both MSRP URIs associated 
with
     an MSRP session to each of the participating endpoints. The 
rendezvous
     mechanism MUST implement mechanisms to provide these URIs securely 
-
     they MUST NOT be made available to an untrusted third party or be 
easily discoverable.

 - The rendezvous mechanism MUST provide mechanisms for the negotiation 
of any
    supported MSRP extensions that are not backwards compatible.

   - The rendezvous mechanism MUST be able to natively transport im: 
URIs or automatically
     translate im: URIs into the addressing identifiers of the 
rendezvous protocol.

To use a rendezvous mechanism with MSRP, an RFC must be prepared 
describing
how it exchanges MSRP URIs and meets these requirements listed here. 
This document
provides such a description for the use of MSRP in the context of 
SIP/SDP.

The Session Initiation Protocol (SIP) meets these requirements for a
rendezvous mechanism. The MSRP URIs are exchanged using the
Session Description Protocol (SDP) in an offer/answer exchange using
SIP. The exchanged SDP can also be used to negotiate MSRP extensions.
This SDP can be secured using any of the mechanisms available in SIP, 
including
using sips to ensure transport security across intermediaries and 
S/MIME for
end-to-end protection of the SDP entity. SIP can carry arbitrary URIs 
(including im: URIs)
in the Request-URI, and procedures are available to map im: URIs to 
sip: or sips: URIs.
 It is expected that initial deployments of MSRP will use SIP as its 
rendezvous mechanism.


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